The present invention is directed, in general, to an optical device and, more specifically, to a mirror or an array of mirrors for use with a micro-electro-mechanical system (MEMS) optical device, and a method of manufacture therefor.
Optical communication systems typically include a variety of optical devices, for example, light sources, photo detectors, switches, cross connects, attenuators, mirrors, amplifiers, and filters. The optical devices transmit optical signals in the optical communications systems. Some optical devices are coupled to electro-mechanical structures, such as thermal actuators, forming an electro-mechanical optical device. The term electro-mechanical structure, as used herein, refers to a structure that moves mechanically under the control of an electrical signal.
Some electro-mechanical structures move the optical devices from a predetermined first position to a predetermined second position. Cowan, William D., et al., xe2x80x9cVertical Thermal Actuators for Micro-Opto-Electro-Mechanical Systems,xe2x80x9d SPIE, Vol. 3226, pp. 137-146 (1997), describes one such electro-mechanical structure useful for moving optical devices in such a manner.
These micro-electro-mechanical systems (MEMS) optical devices often employ a periodic array of micro-machined mirrors, each mirror being individually movable in response to an electrical signal. For example, the mirrors can each be cantilevered and moved by an electrostatic, piezoelectric, magnetic, or thermal actuation. See articles by L. Y. Lin, et al., IEEE Photonics Technology Lett. Vol. 10, p. 525, 1998, R. A. Miller, et al. Optical Engineering Vol. 36, p. 1399, 1997, and by J. W. Judy et al., Sensors and Actuators, Vol. A53, p. 392, 1996, which are incorporated herein by reference.
The mirrors used in these optical devices are typically made up of a material which reflects light with high reflectivity at a desired operating wavelength of the light, for example an operating wavelength ranging from about 800 nm to about 1600 nm for SiO2 optical fiber-based telecommunication systems. Some examples of such reflective materials are gold, silver, rhodium, platinum, copper and aluminum. These reflective metal films typically have a thickness ranging from about 20 nm to about 2000 nm, and are deposited on a movable membrane substrate such as a silicon substrate. At least one adhesion-promoting bond layer is desirably added between the reflective metal film and the substrate in order to prevent the reflective metal film from getting peeled off.
A typical MEMS mirror comprises a metal-coated silicon mirror movably coupled to a surrounding silicon frame via a gimbal. Two torsional members on opposite sides of the mirror connect the mirror to the gimbal, and on opposite sides of the mirror, define the mirror""s axis of rotation. The gimbal, in turn, is coupled to the surrounding silicon frame via two torsional members defining a second axis of rotation orthogonal to that of the mirror. Using the typical MEMS mirror, the light beam can be reflected and steered in any direction.
Commonly, electrodes are disposed in a cavity underlying the mirror and the gimbal. Voltages applied between the mirror and an underlying electrode, and between the gimbal and an electrode, control the orientation of the mirror. Alternatively, an electrical signal can control the position of the mirror magnetically or piezoelectrically.
Turning to Prior Art FIGS. 1 and 2, illustrated is a typical MEMS mirror device and its application. FIG. 1 illustrates a prior art optical MEMS mirror device 100. The device 100 comprises a mirror 110 coupled to a gimbal 120 on a polysilicon frame 130. The components are fabricated on a substrate (not shown) by micromachining processes such as multilayer deposition and selective etching. After etching, the mirror 110, the gimbal 120 and the polysilicon frame 130, are raised above the substrate by upward bending lift arms 140, typically using a release process. The mirror 110 in the example illustrated in FIG. 1, is double-gimbal cantilevered and attached onto the polysilicon frame 130 by springs 150. The mirror 110 can be tilted to any desired orientation for optical signal routing via electrostatic or other actuation, using electrical voltage or current supplied from outside. Typically, the mirror 110 includes a light-reflecting mirror surface 160 coated over a polysilicon membrane 170, which is typically of circular shape. The light-reflecting mirror surface 160 is generally deposited by known thin film deposition methods, such as evaporation, sputtering, electrochemical deposition, or chemical vapor deposition.
Turning briefly to Prior Art FIG. 2, illustrated is an important application of the mirror 110 illustrated in FIG. 1. FIG. 2 illustrates an optical cross connect system 200 for optical signal routing, including an array of mirrors 210. The optical cross connect system 200 shown in FIG. 2 includes an optical input fiber 220, an optical output fiber 230 and the array of MEMS mirrors 210, including a primary mirror 212 and an auxiliary mirror 215. As is illustrated, an optical signal from the input fiber 220 is incident on the primary mirror 212. The primary mirror 212, with the aid of the auxiliary mirror 215, is electrically controlled to reflect the incident optical signal to the optical output fiber 230. In alternative schemes, the input fibers and the output fibers are in separate arrays, and a pair of MEMS mirror arrays are used to perform the cross connect function.
The tilting of each mirror is controlled by applying specific electric fields to one or more of the electrodes beneath the mirror. Undesirable variations in the gap spacing between the mirror layer and the electrode layer, symmetric or nonsymmetric, may alter the electric field for the applied field, which affects the degree of electrostatic actuation and hence the degree of mirror tilting. This in turn alters the path or coherency of light signals reaching the receiving fibers, thus increasing the signal loss during beam steering.
An array of such MEMS mirrors is essentially composed of two layers: a mirror layer comprising the array of mirror elements movably coupled to a surrounding frame, and an actuator layer comprising the electrodes and conductive paths needed for electrical control of the mirrors. One approach to fabricating the array is to fabricate the actuator layer and the mirror layer as successive layers on the same workpiece and then to lift up the mirror layer above the actuator layer using vertical thermal actuators or using stresses in thin films.
An alternative approach is to fabricate the mirror layer on one substrate, the actuator layer on a separate substrate and then to assemble the mating parts with accurate alignment and spacing. The two-part assembly process is described in U.S. Pat. No. 5,629,790 issued to Neukermans et al. on May 13, 1997, which is incorporated herein by reference. This two-part assembly process provides a more robust structure, greater packing density of the movable mirrors, and permits larger mirror sizes and rotation angles, as well as being easily scalable for larger arrays using silicon fabrication processes. The movable membrane in such a MEMS device is preferably made of single crystal silicon, and is typically only several micrometers thick. Such a thin silicon membrane is made, for example, by using the well-known silicon-on-insulator (SOI) fabrication process. The SOI process allows a convenient way of fabricating a thin silicon membrane, and the presence of a buried oxide layer is useful as an etch-stop barrier in photolithographical fabrication of the mirror, gimbal and spring/torsion bar structures. Selected patterned areas of the SOI substrate are etched, e.g., by using chemical etch, reactiveion etch, or a combination of these processes to form the mirror array pattern with cavity structure. The gimbals and the torsion bars are also formed around each mirror. The SOI material and process are described, for example, in Concise Encyclopedia of Semiconducting Materials and Related Technologies, Edited by S. Mahajan and L. C. Kimmerling, Pergamon Press, New York, 1992, p. 466.
Since the movable membrane is typically thin and fragile, with about a 1-10 micrometer thickness for the ease of mirror movement operation, the mirror layer substrate base (for example, SOI material), which carries the mirrors and gimbals, is advantageously made substantially thicker than the movable mirror membrane, thus providing mechanical stability to the structure. The desired thickness of the SOI substrate for MEMS mirror applications is typically in the range of 50-1000 micrometers, preferably 200-500 micrometers.
In the surface-micro-machined optical MEMS devices, such as optical cross-connects, the movable mirrors are often made of poly-silicon membranes, and are coated with a light-reflecting metal such as gold or aluminum on the top surface. The deposition of such metallization films introduces stresses in the mirror, which tends to cause undesirable mirror curving. This may be due to a number of different reasons, such as a film-substrate mismatch in the coefficient of thermal expansion (CTE), a mismatch in the lattice parameter, nonequilibrium atomic arrangement in the film, inadvertent or intentional incorporation of impurity atoms, etc. The presence of such stresses tends to cause a variety of dimensional instability problems, especially if the substrate is relatively thin, as is the case in the MEMS membranes, which are usually only several-micrometers thick. Other examples of the stress caused dimensional problems in the MEMS mirror structure, may include: i) undesirable bowing of the mirror substrate (membrane), which results in a non-focused or nonparallel light reflection and an increased loss of optical signal, ii) time-dependent change in mirror curvature due to the creep or stress relaxation in the reflective metal film, bond layer or the membrane substrate, and iii) temperature-dependent change in mirror curvature due to the altered stress states and altered CTE mismatch conditions in the metal film, bond layer, and membrane substrate materials, with changing temperature.
Turning initially to Prior Art FIG. 3, shown is a graph 300 that illustrates experimental data showing the mirror curvature and temperature dependent change problems arising from the use of single-sided metallization on a Si MEMS membrane. As is evident from FIG. 3, the single-sided metallization produces undesirable mirror curvature as well as a severe temperature-dependent change in curvature, both of which are undesirable for light beam steering applications such as optical cross-connects. In the current example, the primary reason for the curvature formation is most likely the stress caused by the substantial mismatch in the coefficient of thermal expansion (CTE) between the Si membrane (about 4xc3x9710xe2x88x926/xc2x0 C.) and the metallization (about 14xc3x9710xe2x88x926/xc2x0 C.), although the film growth-related stresses may also contribute.
Achieving a flat mirror with a small curvature is essential in order to minimize optical losses associated with such non-flat mirrors. In addition, ensuring a small curvature with a low or negligible temperature dependence of mirror curvature is important, as the optical MEMS mirrors are often subjected to high temperature exposure for the purpose of assembly, packaging and other manufacturing processes, as well as to fluctuations in ambient temperature during operation. One way of correcting such curvature and restoring flat mirror geometry is to employ ion implantation which introduces a compressive stress to cancel out the existing tensile stress in the curved (often concave upward) mirrors.
Such an undesirable mirror curvature in one-side metallized silicon membrane is also seen in the case of the two-part MEMS assembly structure where the mirror layer is, for example, made of the single crystal silicon membrane fabricated from the SOI substrate, and is then subsequently bonded to the electrode layer to form the actuateable MEMS device. However, in this case, both sides of the mirror layer are available for metallization, and the mirror curvature problem can thus be resolved through using a double-layered metallization, i.e., by depositing the same metallization in exactly the same thickness onto both the top and the bottom surface of the silicon membrane, so that the metallization-induced stresses are balanced. Turning briefly to Prior Art FIG. 4, shown is a graph 400 that illustrates experimental data showing the benefits arising from the use of double-sided metallization on the Si MEMS membrane. As can be noticed from FIG. 4, the mirror is substantially flat and contains only a small temperature dependence of curvature.
The presence of two parallel, highly reflective surfaces at the top and bottom surfaces of the silicon membrane, however, introduces multiple light reflections within the silicon membrane, and tends to cause undesirable optical interference and signal loss. The optical interference and signal loss, which is generally referred to as Fabry-Perot (F-P) interferrometric loss, and is more thoroughly discussed in the book by E. Hecht, Optics, 3rd edition, Addison-Wesley, New York, 1998, p. 413-416, is illustrated in FIG. 5. The data in FIG. 5 represents the calculation of F-P interferrometric loss in an optical signal reflected from a 3 xcexcm thick MEMS silicon membrane mirror coated double-side with varying thicknesses of gold metallizations. As is evident from FIG. 5, the presence of Fabry-Perot cavity, with the light beam repeatedly reflecting from the two bounding metallization mirror layers (double-sided Au metallizations), introduces non-uniform wavelength-dependent F-P interferrometric loss, which is especially significant for the thinner metallizations. If the metallization thickness is increased, e.g., to about 80 nm as in FIG. 3, the F-P loss is progressively reduced, although not completely.
The use of the thicker metallization layer, however, poses another problem of adding too much weight to the movable membrane. Gold, which as previously mentioned may act as one metallization material, has a density of 19.32 g/cm3, which is more than about 8 times heavier than the Si membrane material (densityxe2x88x922.33 g/cm3). Increasing the gold thickness from about 33 nm to about 80 nm on both sides of the 3 xcexcm thick Si membrane would increase the overall weight of the movable membrane by more than about 25%. Such an increase in the mass of the movable membrane is not desirable, as it affects the dynamics of mirror movement and slows down the response time substantially, and further, the resonance frequency of the mirror will be reduced, increasing the sensitivity to external mechanical perturbations during operation. In addition, the increase of mass on the MEMS spring regions raises the stiffness of the spring, and hence the actuation voltage to move the mirror by a given displacement or tilt angel will have to be increased significantly.
It is desirable to have a double-side metallized MEMS mirrors so that the flatness of the mirrors is maintained, yet at the same time such Fabry-Perot interference loss is prevented or minimized. Accordingly, what is needed in the art is a micro-electro-mechanical system optical device, and a method of manufacture therefore, that does not encounter the problems associated with mirror curvature and F-P interferrometric loss, as experienced in the prior art electro-optic MEMS devices.
To address the above-discussed deficiencies of the prior art, the present invention provides a mirror, or array of mirrors, for use in a micro-electro-mechanical system (MEMS) optical device. The mirrors include a mirror substrate having a loss-reducing layer located over a first or second side thereof, and a light reflective optical layer located over the loss-reducing layer.
In another aspect, the present invention includes a method of manufacturing the mirror. The method includes (1) providing a mirror substrate having a loss-reducing layer located over a first or second side thereof, and (2) forming a light reflective optical layer located over the loss-reducing layer.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.